To fabricate a Schottky barrier diode (SBD), the region outside the active regions of the device is first etched to the high-resistivity epitaxy layer before fabricating ohmic contacts on both sides of Schottky contacts, where the Schottky contacts are the anodes and the ohmic contacts are the cathodes. The greatest defect of the design is that the spacing between electrodes determines the forward current and the reverse breakdown voltage. As the spacing is shortened, the forward current is increased whereas the reverse breakdown voltage is lowered. Contrarily, when the spacing is increased, the reverse breakdown voltage is increased whereas the forward current is lowered. Thereby, according to the prior art, trade-offs occur between forward current and reverse breakdown voltage and hence bringing inconvenience for circuit design. In order to overcome the above defect, according to the prior art, the SBD is coupled to a high electron mobility transistor (HEMT). As shown in FIG. 1, in forward bias, the HEMT is turned on; in reverse bias, the HEMT is turned off. By using this method, the SBD is protected from breakdown in reverse biases. Nonetheless, this method requires coupling two independent devices. In circuit design, it requires more area. Besides, in high-speed switching, the overall speed will be lowered, resulting in degradation of device performance.
A HEMT is generally classified into two modes: the depletion mode and the enhancement mode. According to the prior art, the channel of a depletion-mode HEMT is injected with fluorine ions (F−) so that the 2-dimensional electron gas (2DEG) in the injected region is raised above the Fermi energy level and forming an enhancement-mode HEMT. Nonetheless, no matter in which mode, the HEMTs according to the prior art still suffers from the trade-off between current collapse effect and drain to source breakdown voltage. In addition, for an active device, it is considered how to reduce the surface leakage current.
In the past, silicon dioxide (SiO2) is generally adopted as the surface passivation film of a HEMT for increasing it breakdown voltage as well as reducing the surface leakage current. By using the deep traps formed by using SiO2 as the passivation film interface, the electrons are trapped and thus increasing the breakdown voltage. Nonetheless, the deep traps induce the problem of slow current recovery when the drain of the HEMT is given a high voltage bias in off-mode (which by means the drain to gate is given a high voltage reverse bias) is switched to an on-mode (which by means the drain to source is given a high voltage forward bias). According to the prior art, there have been many discussions on reverse recovery current. Nevertheless, there is no publication focused on analysis or discussion of current collapse effect. FIG. 2 shows a schematic diagram of current collapse effect of a GaN SBD. When a device is reversely biased, the current is extremely small and approaches to zero. At this moment, as a forward bias is applied, the current cannot increase as soon as the voltage. It requires a delay time. Thereby, under high-speed operations, the operating speed of the device is reduced.
Accordingly, according to the prior art, there still exist the problems of inability in optimizing the forward current and the reverse breakdown voltage and in optimizing the forward recovery current and the surface leakage current.